As much as I'd like to believe it — especially in these cases — the customer is not always right. In fact, sometimes the customer is downright dunderheaded, as I was in a couple of recent instances involving rental cars. Rookie mistakes both, and they cost me — in one case time, in another money.

On a rainy Sunday morning last month, I returned a three-day rental car to the Des Moines airport. The rate was about $43 a day for a compact car. I had picked up the rental car at 9 a.m. Friday and had said on my reservation that I would return it at 10 a.m. Sunday.

I'm not sure what I was thinking — not sure I should even dignify it by calling it thinking — because my flight didn't leave until 1:30 p.m., and I wasn't planning to spend quality time in the airport.

As I zipped through the heart of Iowa on my 130-mile trip back to Des Moines, I congratulated myself on being right on the money, time-wise, despite heavy rain and having to stop to fill the car.

All systems were go. But as soon as I reached the rental-car counter, the "go" turned to "Oh. Oh. Oh. Oh." The agent frowned and said that because I was returning the car late, he would have to charge me for an extra day. When making the reservation, I had put down a return time that was 93 minutes before I showed up with the car. The agent was perfectly pleasant, made appropriate sympathetic sounds and put the extra $42.53 on my credit card.

But the rental company's website, I learned, said the company should prorate, not charge an extra day. I spent almost 50 minutes on hold, first with one agent, then another and in the end the company credited back the extra charge.

"But I was late," I said. And she replied, "Well, you weren't that late." (She had no idea what I do for a living and I didn't tell her.)

No need to argue with success, especially in light of my other rental-car blunder that boosted my rate by almost $400.

In May, I rented a much nicer car than I would usually reserve for a trip that started in Boston and ended in the Washington area. (Why nicer? Hubs hates tiny cars; he's at least 9 inches taller than I and needs leg room, an alien concept to someone who must get petite-length slacks shortened.)

Our multistop itinerary took us through several states and multiple adventures. On a Sunday morning, again proud of myself for getting to the airport on time, we arrived at the rental-car drop-off, and the agent checking us in said, "Here's your receipt for $903."

Huh? The car as I reserved it was going to be about $525, hefty because of its size and because it was a one-way rental. When I asked, the manager said I was returning the car to the wrong location. Before I could sputter, he said, "You said you were returning it to Washington Dulles." I was standing in the rental-car company counter at Baltimore Washington Airport, 58 miles away.

And then I remembered that our plans had changed. We had found an earlier flight out of BWI, but I hadn't changed the rental-car drop-off location. I did call the rental-car company when I returned to L.A., and it refunded me $118, which helped ease the pain slightly. I also cursed myself for making these mistakes on my dime, not The Times' dime, although explaining to the boss would have been easier than explaining to the hubs.

I asked Laura Bryant, spokesperson for Enterprise Holdings Inc., which includes the Enterprise, National and Alamo brands, for some insight. In the end, she said, it comes down to understanding that when you do what I did, you're throwing a monkey wrench into fleet management.

Many factors affect car rental costs, she said — what else is going on (Conventions? World Series?), whether it's a long weekend (my ill-fated BWI return was Memorial Day weekend) and whether it's peak travel season (yes). The algorithms that set costs are as complex as the management of the fleet.

"The thing is to put yourself in the place of the other customer … who's looking for your car at that other airport," Bryant said. In a small market, there may be less flexibility.

What I did also probably inconvenienced other people. Dumb and rude — one of my least favorite combinations, along with wasting time and money.

The solutions are as obvious as what Miss Yee told us in the third grade: Double-check your work. Try not to make mistakes.

Some days I'm amazed I made it out of the third grade, and those were two of them.

Have a travel dilemma? Write to travel@latimes.com. We regret we cannot answer every inquiry.

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